This invention relates to a simplified transmission system for color television signals having improved signal-to-noise (SN). In magnetic recording of television signals by video tape recorders, the record or playback head may occasionally fail to come into contact with the magnetic oxide coating because of variations in tape tension, build-up of dirt on the heads and the like. This may cause a reduction in the amplitude of the signal transduced from the tape to the playback head, and may result in objectionable distortion. It is known to frequency-modulate a carrier with the video information to be recorded. Such frequency modulation of the signal translates amplitude changes of the video signal into frequency changes of the carrier. An amplitude limiter coupled to receive the frequency-modulated signal strips away amplitude variations resulting from imperfect head contact, and the frequency-modulated signal when demodulated has an improved signal-to-noise compared with the case of direct recording.
When color television signals encoded in a standard NTSC manner including luminance components and chrominance components quadrature-modulated onto a color subcarrier are recorded, the total frequency bandwidth of the video signal is large. When it is desired to record such an NTSC signal, it is found that the total bandwidth of the NTSC signal is so large that the sidebands of the frequency-modulated carrier extend over a greater frequency band than can be encompassed within the FM channel of the recorder. Consequently, the "color under" system has been used in the past. In this system, the color subcarrier, quadrature modulated with chroma components, is directly recorded at a low frequency on the same tape track with an FM carrier modulated by video luminance information. To improve linearity, the directly recorded chrominance information is recorded with the aid of a bias signal. To prevent interaction between the bias signal and the frequency-modulated carrier, the FM carrier is often used as the bias signal. While such an arrangement allows recording of a color television signal on a single track of a video tape recorder, certain problems exist, such as poor SN of the chrominance signal, crosstalk between the two quadrature-modulated color signals, and limited frequency bandwidth which necessitates reduction of the desired bandwidth in either the chrominance or luminance information, or possibly both. Furthermore, the FM luminance carrier cannot be modulated to the maximum possible amount because maximum modulation drives the recording medium into saturation, adding distortion to the directly recorded chrominance information.
In order to improve the quality of the television signal to broadcast standards, the luminance information may be recorded on a first track of the tape by the use of a frequency-modulated carrier, while at the same time recording the quadrature-modulated chrominance information onto a second track of the tape adjacent the first. The chrominance information is modulated onto a frequency-modulated carrier for improved signal-to-noise. It has been found, however, that broadcast quality may not be achieved even in such a system using two wideband channels for the recording of the video information. Furthermore, it has been found that cross-modulation occurs between the two components of the chrominance signal.
In order to obtain improved characteristics for a two-channel tape recorder or other transmission system, it is desirable to reduce the frequency of the signal modulating the chrominance channel. Each of the baseband color-difference signals alone has a lesser bandwidth than does the quadrature-modulated signal. The frequency bandwidth of the signal modulating the chrominance channel may be reduced by alternately modulating the frequency-modulated carrier in the chrominance signal channel with one of the two chrominance signals representing the chrominance information. For example, if the chrominance information is represented by I and Q signals, where the I signal has a frequency bandwidth of 1 MHz and the Q signal has a frequency bandwidth of 0.5 MHz, each of these signals is alternately modulated onto the carrier for coupling into the channel. Alternation, however, results in a loss of I signals during that interval in which Q signals are being carried through the system, and similarly Q signals are lost during that interval in which I signals are being processed. Thus, there is a loss in signals similar to that which occurs in a SECAM system. In the SECAM system, the line-to-line alternation of the chrominance information results in a reduced vertical chrominance resolution which degrades the picture. U.S. patent application Ser. No. 124,107 filed Feb. 25, 1980 in the name of Dischert et al describes a two-channel system in which baseband luminance is transmitted in real time while baseband chrominance is time-compressed by delay lines. Each baseband chrominance signal for transmission is clocked into a delay line at a low clock rate and is clocked out at a higher rate, thereby creating the time compression. (At the receiving end, a complementary operation takes place). Ordinarily, there are two baseband color-difference signals to be processed. Since they are time-division multiplexed onto the single chrominance channel, they must pass through the channel sequentially. Consequently, the aforementioned Dischert et al system used four delay lines and a system of switches for coupling signals to and from the delay lines and also for coupling high and low clock signals to the delay lines for controlling the writing-in and reading-out rates.
It is necessary to use two delay lines per channel to accomplish the desired compression because the write-in and read-out rates are different, yet the CCD delay lines ordinarily used will write in while operated at the clock-out rate and will read out while operated at the write-in rate.
The CCD delay line is ordinarily an integrated circuit which is made by processing techniques which form an entire delay portion and its associated controlled clocking drive and internal control circuits simultaneously in a batch process. It has been found by at least one manufacturer (Fairchild) that it is advantageous to form two independent delay portions and one controlled clock drive and internal control system in each IC. Thus, each IC contains one control circuit and two independent delay lines. Since each of the two delay lines in the IC are controlled from the same control circuit, they are clocked in parallel and operate at the same time.
It is desirable to reduce the cost of a time-compressed signal processing system by making use of the two delay lines in each IC which are controlled in parallel.